Calculate the number of mda regions by multiplication
Calculate the number of mda regions by multiplication

Make it the derived quantity and NUM_PRIMARY_MDA_REGIONS the base quantity.
because multiplication is easier to reason about than division.

Use NUM_PRIMARY_MDA_REGIONS constant wherever it is applicable.
In order to avoid the "magic number" problem.

Omit PER_MDA_REGION_COPIES. It isn't used quite enough to justify a
separate constant, but add some comments to NUM_MDA_REGIONS to better
explain.

Signed-off-by: mulhern <7b51bcf507bcd7afb72bf8663752c0ddbeb517f6@redhat.com>

stratis-storage/stratisd,stratis-storage/stratisd,stratis-storage/stratisd